To manufacture a sandwich panel, for example, a conventionally used forming method employs primary forming (pre-forming) using extrusion, in combination with secondary forming using blow (or vacuum). According to such a forming method, an extruded molten resin is directly subjected to blow molding (or vacuum forming). This method thus makes it possible to form a sandwich panel without posing a technical problem such as non-uniform heating, which would otherwise be caused by reheating of an already formed resin. Particularly in this forming method, blow molding (or vacuum forming) is performed by causing an extruded molten resin to droop downward as it is, and then clamping the vertically extending resin. Therefore, it is not necessary to support the molten resin until clamping performed in the secondary forming, unlike in the case of, for example, extruding a resin laterally. It is thus possible to feed a resin in a non-contact state from an extrusion die. Such a forming technique is disclosed in, for example, Patent Documents 1 to 3.
Patent Document 1 discloses a method for forming a sandwich panel comprising two panel members and a foamed core member sandwiched between the two panel members wherein a pair of sheet-shaped resin s are extruded downward from an extrusion die to be disposed between a pair of split molds, while the foamed body having been already formed is disposed between the pair of sheet-shaped resins, and then, the split molds are clamped to carry out blow-molding.
More specifically, each of the pair of sheet-shaped resins extruded from an extrusion head and a decorative sheet are attached to each other using pressure by a pair of pressing rollers to form a decoration panel consisting of the sheet-shaped resin on its inner side and the decorative sheet on its outer side, and then, disposed between the split molds.
Patent Document 2 discloses a blow-molding technology wherein each of two synthetic resin sheets are extruded from a die to pass through a pair of mirror finished heat rollers so that its surface is made mirror finished, after which it is fed to a blow-molding mold, and then, pressurized air is supplied between two sheets to deform two sheets so as to conform to a forming space.
More specifically, a gap between the mirror finished heat rollers is narrowed to be slightly thinner than a thickness of a polypropylene sheet, while the mirror finished heat rollers are heated to 130° C., so that the sheet is thermally pressed by the fact that it is passed through the mirror finished heat rollers, whereby pressing marks on the surface of the sheet are distinguished, so that the surface of the sheet is modified so as to conform to the surface of the roller.
Patent Document 3 discloses a blow-molding technology wherein each of sheets is extruded from at least two dies connected to an extruding machine, after which each of sheets is sandwiched by a pair of rollers, while at least its surface is heated to extend its wrinkles to be glazed, and then, the at least glazed two sheets are pulled to be fed into a blow-molding mold, whereby the mold is clamped so that two sheets are attached to each other.
More specifically, both glazed sheets are fed into a blow-molding mold almost simultaneously by controlling a pulling speed of the sheet, or regulating the number of rotations of a screw of the extrusion machine, in accordance with a state of a draw-down of each sheet.
In this case, a draw-down phenomenon is defined to be the one in which an upper portion of a molten sheet becomes stretched, and thus, thinned more than a lower portion thereof due its own weight as time elapses, while a neck-in phenomenon is defined to be the one in which a width of the molten sheet becomes contracted in its widthwise direction, and thus, narrowed due to the draw-down phenomenon.
In each of the forming techniques disclosed in Patent Documents 1 to 3, a molten resin is allowed to pass through between a pair of rollers, before the molten resin extruded downward is formed within a mold. However, the pair of rollers disclosed in Patent Document 1 is merely pressure rollers for pressure-bonding a sheet-shaped resin extruded from an extrusion head to a decorative sheet. In the techniques disclosed in Patent Documents 2 and 3, on the other hand, a gap between the pair of rollers is set smaller than a thickness of a sheet-shaped molten resin. In addition, a temperature of the rollers is set close to that of the sheet-shaped molten resin. Under such conditions, the sheet-shaped resin is allowed to pass through between the pair of rollers to thereby be formed. It is thus possible to provide a mirrored or glossy surface of the sheet. In these conventional forming techniques, however, the molten resin extruded prior to secondary forming is caused to droop downward as it is. As a result, the following technical problems arise.
The first problem is that draw-down or neck-in, which occurs in a molten sheet, makes a thickness of the sheet prior to forming in a mold uneven in an extruding direction or a width direction of the sheet. When the sheet is finally formed by secondary forming using blow or suction (vacuum), the uneven thickness in the extruding direction of the sheet before the secondary forming affects a thickness of the finished sheet after the secondary forming. To cope with this problem, for example, when, during forming of the sheet, an extrusion speed of the sheet is changed to increase according to the draw-down as the forming proceeds, a thickness of the sheet corresponding to a later stage of the forming can be increased toward an upper part of the sheet. Therefore, it seems possible to cope with the thinning of the upper part of the sheet caused by the draw-down. However, the change in the extrusion speed of the sheet causes a change in an extrusion pressure of the sheet. As a result, a swelling in the molten resin extruded from an extrusion slit changes, which rather accelerates the unevenness of the sheet thickness. Note that Patent Document 3 discloses controlling a pulling speed of a sheet or adjusting a rotation speed of a screw of an extruder according to the draw-down of each sheet. However, this does not suppress or eliminate occurrence of the draw-down of each sheet. In this case, assuming that the draw-down occurs in two sheets, the pulling speed of the sheets is controlled or the rotation speed of the screw of the extruder is adjusted, and consequently, both of the sheets are merely supplied into a mold almost at the same time.
It is possible to prevent such draw-down or neck-in to some extent by employing, as the resin used for a sheet, the one having a predetermined or larger MFR value or melt tension value. In this case, however, materials that can be used are limited, which is not practical. Particularly when forming a thin sheet, the larger the MFR value, the better. For these reasons, limiting the MFR value may not solve the problem.
The documents that describe the related art are listed below.    Patent Document 1: JP 2000-218682 A    Patent Document 2: JP 3-27922 A    Patent Document 3: JP 11-5248 A